Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Second Person, Spoken to Ty
Given the seriousness of her situation, it makes sense that Gemma would be the one telling us the story. However, there's a twist—her audience isn't just us as readers, but Ty. While he's likely never going to get to read it (he's locked up until his trial and then will likely be locked up for a long time), Gemma tells us the story in her own voice while simultaneously addressing Ty directly, using the pronoun "you" throughout the book.
This has an interesting effect because we're not only getting her account of the events, but the characters of Ty, her friends, and family are filtered through her eyes. Ty may be the primary person being addressed, not us, but we still get to feel like we're eavesdropping on their correspondence, experiencing the honesty and intimacy of Gemma's memories.